bug#56393: Actually fix the long lines display bug

[Eli Zaretskii]

> Cc: gerd.moellmann@gmail.com, larsi@gnus.org, 56393@debbugs.gnu.org
> Date: Sat, 09 Jul 2022 12:20:38 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > I guess moving the calculation of the restriction into start_display 
> > should fix that problem.  Then it->narrowed_begv will be 0 for strings.
> 
> OK.  It wasn't celar to me what you intended to do, because
                 ^^^^^
I meant "clear" here, sorry for the typo.